1. Field of the Invention
The present invention relates to an image display device for performing preferable halftone expression by eliminating flicker and image unevenness and a display method of the halftone expression.
2. Discussion of the Related Art
A liquid crystal display (LCD) device and a plasma display device have been recently noticed as power-saving, slim and lightweight image display devices. In case of these display devices, image display is normally performed by a direct driving system in accordance with a digital image signal. Moreover, to display a halftone monochromatic image other than white and black or a color image in accordance with three primary colors of red (R), green (G) and blue (B), gray level display referred to as “halftone expression” is necessary. Therefore, the number of gray levels is decided by the number of bits of an image signal to be used and the necessary number of bits of the image signal increases as the number of gray levels increases.
For example, in case of a LCD device, it is difficult to express more than 256 (=28) gray levels because a source driver commonly used only having 8-bit. To display more gray levels, it is necessary to develop and use such as a 12-bit source driver. In this case, however, because a circuit size increases compared to an 8-bit source driver, a problem occurs that the cost of the source driver increases.
Therefore, to express more gray levels without increasing the number of bits which can be processed by a source driver, a method called “Frame-Rate-Control (FRC) method” is proposed. The FRC method sets the number of bits supplied to the source driver to a value equal to or less than the number of bits of an input image data and applies the frame-thinning control corresponding to an insufficient number of bits for an insufficient number of gray levels. For example, 10-bit input image data is divided into four 8-bit frame data. And these frame data is supplied sequentially to an 8-bit source driver to display gray levels for 10 bits using the 8-bit source driver.
However, the FRC method has a problem that it is difficult to increase the number of frames (thinning number of frames) displayed by one input data because flicker or uneven image occurs. To solve this problem, an “error-diffusion frame-thinning system” is proposed in which the difference between voltage of a gray level to be displayed on a certain pixel and the voltage of the nearest gray level which can be displayed predetermined hardware is regarded as an “error” and the error is reflected (diffused) on the voltage of gray level of pixels present around the pixel.
As one of FRC gray level methods for achieving increase of display gray levels above described, there is the “PICTURE DISPLAY METHOD AND PICTURE DISPLAY DEVICE USED FOR THE METHOD” (Japanese Patent Laid-Open No. 2001-34232). The above method and device are an image display method and device for displaying a monochromatic image having a gray level resolution larger than the reproducing capacity of R, G and B of a color display panel in which a unit pixel is constituted by a combination of three pixels of R, G and B in accordance with the gray level expression corresponding to the input bits of the monochromatic image by using the FRC gray level method when displaying a monochromatic image by the color display panel.
FIG. 1 is a block diagram of a LCD device 100 disclosed in Japanese Patent Laid-Open No. 2001-34232. The LCD device 100 is provided with a color LCD 101 for displaying an image by liquid crystal, a backlight section 102 serving as a light source of the color LCD 101, a data processing section 104 for performing predetermined data processing, a source driver 103 for driving the color LCD 101, and an interface (I/F) 105 for capturing input image data into the data processing section 104.
FIGS. 2A and 2B are locally enlarged views of the color LCD 101. As shown in FIG. 2A, the display screen of the color LCD 101 is constituted so that R-pixel, G-pixel and B-pixel are horizontally lined up when using a color filter. That is, R-pixel, G-pixel and B-pixel are arranged in accordance with “stripe arrangement”. Color display according to image data values of R, G and B is normally performed through the R-pixel, G-pixel and B-pixel. In the case of a conventional invention, a monochromatic image is displayed as described below.
As shown in FIG. 2B, the LCD device 100 uses R-pixel p1, G-pixel p2 and B-pixel p3 as an unit pixel p to display a monochromatic image. In this case, the unit pixel p is constituted by the R-pixel p1, G-pixel p2 and B-pixel p3 when using a color filter. Therefore, the set number of a brightness value which can be displayed by one unit pixel p becomes three times larger than the set number of the brightness value which can be displayed by each of the R-pixel p1, G-pixel p2 and B-pixel p3. That is, by setting the brightness range to 1/3, it is possible to increase the number of gray levels of a display image.
Then, as a specific example, the FRC performed by the data processing section 104 is described when 10-bit monochromatic-image data is supplied to the interface (I/F) 105 by assuming that R-pixel p1, G-pixel p2 and B-pixel p3 respectively perform 8-bit display by the 8-bit source driver 103.
In this case, because input image data is 10-bits and the data to be processed by the source driver 103 is 8-bits, the difference between the bits is equal to 2. Therefore, the number of frames in the frame cycle under the FRC becomes 4 (=22). Therefore, 8-bit image data values are successively displayed by each frame from first to fourth frames for each of the R-pixel p1, G-pixel p2 and B-pixel p3.
The data processing section 104 first divides 10-bit monochromatic-image data (original data) into R data, G data and B data. The above division is performed by referring to the conversion table shown in FIG. 3 (numerical notation in FIG. 3 uses decimal numbers). For example, when the original data is “0”, then “0” is distributed to R data, G data and B data. When the original data is “10”, then “9”, “9” and “10” are distributed to R data, G data and B data respectively. Thus, 10-bit R data, G data and B data are generated from 10-bit monochromatic-image data (original data).
Then, because the R data, G data and B data thus generated are respectively 10 bits (1,024-gray level expression), they are distributed to 8-bit data (256-gray level expression) using four frames, that is, 8-bit “frame data”. Dividing to the frame data is performed by referring to the conversion table shown in FIG. 4. The numerical notation in FIG. 4 also uses decimal numbers.
That is, 10-bit R data, G data, and B data (0–1023) are converted into 8-bit frame data (0–255) for each of first to fourth frames. The above mentioned corresponds to the fact of constituting one frame cycle by four frames generated in time series in the FRC gray level method. Moreover, the above mentioned corresponds to the fact of using 8-bit frame data included in each of four frames to display a group of 10-bit monochromatic-image data (original data) values by a pixel p. The R-pixel p1, G-pixel p2 and B-pixel p3 are driven in accordance with the frame data thus generated and an image constituted by the pixels p1 to p3 is displayed by the pixel p.
As described above, the conventional LCD device 100 for expressing halftones by using the FRC gray level method shown in FIGS. 1 to 4 makes it possible to express gray levels (2K gray levels) corresponding to K bits of input image data by a J-bit source driver which can express 2J gray levels by setting the number of frames in one frame cycle on accordance with the difference N (=K−J) between the numbers of bits when displaying K-bit (K is positive integer of K>J) input image data by a J-bit (J is positive integer) source driver to 2N and distributing K-bit input data to 2N J-bit frame data values.
However, because the number of frames during a frame cycle of FRC is set to 2N, the frame cycle becomes extremely long as the difference N between the numbers of bits increases. As a result, flicker and image unevenness peculiar to the FRC gray level method are generated and the image quality is deteriorated instead.
The present invention is made in view of the above situation and its object is to provide an image display method and an image display device for expressing preferable halftones by using the FRC method and capable of preventing flicker and image unevenness.
It is another object of the present invention to provide an image display method and an image display device for expressing halftones by using the FRC method and capable of keeping the number of frames during a frame cycle at 2N or less when the difference between the number of bits of input image data and the number of bits of a driver is equal to N.
Still other objects of the present invention not described in this specification will become more apparent from the following description and the accompanying drawings.